Combinational simulations of free energy of polymer solution

It is conceptually proposed that the total entropy of polymer solution is contributed from two distinct parts: the positional and the oomformational. The former can be represented analytically, while the latter can be simulated with the random self-avoiding walk model on the simple cubic lattice for multichain systems. The obtained results indicated that both the conformational entropy and the mixing heat are consistent with the scaling laws wry well.     

COMPUTER SIMULATIONS ON UNPERTURBED CONFIGURATIONAL DIMENSIONS OF POLY (METHYL ACRYLATE)

A full-relaxation optimization of molecule and the popular MM2 force field are em-ployed to obtain the geometry parameters and the conformational energy surface of a mesoor a racemic dyad of poly(methyl acrylate) (PMA) with a specified carbonyl-bond orien-tation in side-groups. It is found that the conformational energy maps calculated hereconsiderably differ from those calculated with the rigid molecular model as reported in theearlier studies. The g~- state cannot be omitted in the obtained contour maps. Two impor-tant conformers tg~- and g~(-t) with energy minima were newly detected for a racemic dyad.The analysis on the conformations with energy minima confirmed that the ester groupsare not always perpendicular to the plane defined by the two adjacent skeletal bonds andmay change their relative orientations to meet the requirement of lower energies duringthe conformational state transition. Instead of the early way of adjusting the interactionenergy parameters to fit the experimental data, we attempt to predict unperturbed chaindimensions via the reliable force field and the configurational statistical mechanics. Theproposed scheme with three rotational states identified from the contour maps allowed usto satisfactorily reproduce the experimental dimensions of random PMA chains.     

CONFORMATIONAL CHARACTERISTICS OF POLY(ACRYLIC ACID) AND POLY(METHACRYLIC ACID)

A full-relaxation optimization of molecule and the Dreiding force field are employed toobtain the geometry parameters and the conformational energy surfaces of meso or racemicdyad of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA). Three differentcarbonyl-bond orientations of side-groups resulted in the differences in depth of potentialwells in their energetic contours for a meso or a racemic dyad. These discrepancies areinterpreted as a result of various fine structures corresponding to grid search conformationsas well as thereby different interactions. The analysis on the most stable conformationsof PMAA confirmed that the ester groups are nearly perpendicular to the plane definedby the two adjacent skeletal bonds but may possibly change their relative orientations tomeet the requirement of lower energy during the conformational state transition. For eachpolyme, two global energy maps of a meso and a racemic dyad were finally constructedfrom the superposition of energy data for the three kinds of side-group orientations by theBoltzmann factors. From an ensemble average, the proposed scheme with three rotationalisomeric states (RIS) allowed us to access the experimentally unperturbed dimensions ofPAA chain via the configurational statistical mechanics. Although the calculation wasbased on the short-range, local interactions, it was interested to note that the experimentalcharacteristic ratios just fell within the range calculated for atactic chains.